Field of the Invention
The invention relates to a multiplex drive system for a liquid crystal display and more particularly to such a drive system in which pulsed and synchronized DC voltages are applied to corresponding segments and to base conductors of display characters for producing an input character display while inhibiting non-input displays.
A liquid crystal display in one form may comprise two parallel plates separated by a sufficient distance to accomodate a layer of liquid crystal material. The parallel plates may be coated with transparent conductors comprised of, for example, tin oxide, on the inside surfaces of the plates. Liquid crystal materials are classified according to the arrangement of their molecules into three categories, namely, nematic, cholesteric, and smectic.
Liquid crystals are normally quiescent and essentially transparent in the absence of drive voltages. Whenever drive voltages of sufficient amplitude and duration are applied to conductors on both sides of the material, the electric field created between the conductors alters the molecular arrangement of the liquid crystal materials altered. As a result, light passing through the opague liquid crystal material is forward scattered to form a contrast with the liquid crystal material which remains quiescent. When the drive voltages are removed, the material returns to its original transparent condition after a finite period of time. If the drive voltages are reapplied within the finite period, the display continues without interruption. Generally cyclical drive voltages are used for that purpose.
Liquid crystal displays may be transmissive or reflective. A transmissive display, as shown in FIG. 1, forward scatters light passing from the back plate through the transparent front plate. A reflective display as shown in FIG. 2, forward scatters light as it passes through the front plate to the back plate. The light is reflected from a back plate mirror.
In one type of display, each segment of a character is driven by an individual drive voltage. Thus a liquid crystal display with eight positions for displaying a digit (character) at each position having 7 segments per digit, requires 56 conductors for 56 drive voltages applied to individual segments, and 1 conductor for the drive voltage applied to base conductors. Base conductors usually have a configuration which is approximately coextensive with the circumferential configuration of the combination of character segments. A back single conductor is provided for each combination of segments at a display position.
AC signals have been used as drive voltages to avoid non-reversing DC current flow between front and back conductors. Continuous currents through the liquid crystal material separating the conductors may cause a degradation of the performance of the material. However, the use of AC signals requires somewhat complex circuitry. In addition, because of the relatively high peak to peak voltages, additional restrictions are placed on the AC drive circuitry.
It would be preferred if drive voltages could be supplied by reversing DC voltage levels. In that case, standard digital logic circuitry could be used to produce a drive voltage without the necessity for complex circuitry. However, precautions must be taken to prevent prolonged DC current from passing through the liquid crystal material. The present invention provides such a liquid crystal drive system display that avoids the DC current problem.